Strong consumer interest in home satellite TV reception has motivated research by TVRO system manufacturers into methods for obtaining better performing, more cost-effective, earth station terminals. A TVRO terminal is one component of a typical satellite television system which includes four major components: (1) a television studio where TV signals originate; (2) an up-link station which transmits the TV signal into space; (3) a communications satellite in geostationary orbit which receives the up-linked signal and retransmits it to earth at a down-link microwave frequency; and (4) a TVRO terminal to receive the down-linked microwave signal and convert it into audio and video information for display.
The TVRO earth station terminal itself includes six major components: (1) a directional receiving antenna directed at the desired satellite; (2) a low-noise preamplifier (LNA) mounted directly on the directional antenna; (3) a frequency down-converter; (4) a satellite TV receiver; (5) a VHF remodulator; and (6) a conventional VHF television set.
At the present time, communications satellites rebroadcasting TV signals from geostationary orbits in the Clarke belt are spaced apart by approximately four degrees of longitude. Such close angular spacing places severe requirements on the TVRO earth station microwave antenna. In order to satisfactorily discriminate against interference from satellites that are adjacent to the satellite being received, antennas having high directivity and correspondingly narrow beamwidths are required. Satisfying these requirements with conventional parabolic "dish" antennas dictates the use of reflectors having very large diameters. Accordingly, reflecting dishes having diameters as large as 15 feet are not uncommon in home TVRO satellite systems.
The problem of discriminating against interference from adjacent satellites will soon be exacerbated. Proposals are currently pending before the Federal Communications Commission to increase the number of operating TV satellites by reducing their angular separation to approximately two degrees of longitude. With 2.00 degree longitudinal separation, earth stations near the equator would "see" adjacent satellites directly overhead separated by only 2.35 azimuthal degrees; and those near the poles would "see" the satellites near the horizon separated by azimuthal angles of only slightly more than 2.00 degrees. Serious questions arise as to whether even 15-foot diameter dishes will be sufficiently directive to satisfactorily discriminate between adjacent satellites having such close angular spacing. Clearly, there is considerable current and future need for a small, cost-effective, microwave antenna system that is highly responsive to signals arriving from a primary receiving direction but which can effectively nullify signals and noise arriving from another direction that differs from the primary receiving direction by only a very small angle.